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Coagulation and Flocculation at Water Treatment Plants

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  1. Coagulation and Flocculation at Water Treatment Plants WQT 131 Water Works Operation III Water Treatment Chapter 4 Coagulation and Flocculation Lecture 3

  2. Week 3 Objectives Reading assignment: AWWA 2003. Water Treatment, Principles and Practices of Water Supply Operation, Third Edition, American Waterworks Association, ISBN 1-58321-230-2 Chapter 4 Coagulation and Flocculation • Understand conventional treatment to remove turbidity • Understand turbidity and its primary constituents • Explain coagulation chemistry • Understand mechanism of function involved in coagulant aggregation • Explain flocculation chemistry • Understand mechanism of function involved in flocculent aggregation • Understand the role of pH, alkalinity, turbidity, temperature on coagulation and flocculation application

  3. Key Words • Coagulation: adding and rapid mixing of chemicals to remove particles from water. (flash mixing) • Flocculation:adding and slow mixing of chemicals and particles to create flocs that settle out of water. • Turbidity: suspended, dissolved, and colloidal particles in pretreated water that need to be removed to optimize treatment efficiency. • Suspended Solids: particles held in suspension by the natural action of flowing waters. • Colloidal Solids: fine silt that does not settle out of water but remain in suspension. • Dissolved Solids: organic or inorganic molecules that are dissolved into the aqueous phase.

  4. Which of the following is the main purpose of the coagulation/flocculation process? a. to remove turbidity b. to soften the water c. to add oxygen d. to disinfect.

  5. The most important raw water constituent for a surface water plant is: a. temperature b. hardness c. turbidity d. pH

  6. WQT 131 Water Works Operation III Water Treatment Chapter 4 Coagulation and Flocculation at Water Treatment Plants “Ironically, it is easier to clean up dirty water than to make clean water cleaner. The reason is because particles must collide before they can stick together to make larger flocs. More particles means more collisions.”

  7. Water Treatment

  8. Conventional Treatment • Conventional Treatment– common treatment steps used to remove turbidity from the initial source water. • 1. Coagulation • 2. Flocculation • 3. Sedimentation • 4. Filtration Rapid Mixing Slow Mixing Settling Cleaning

  9. Turbidity • Turbidity– particles (sand, silt, clay, bacteria, viruses) in the initial source water that need to be removed to improve treatment. • 1. Suspended Solids • 2. Colloidal Solids (~0.1 to 1 mm) • 3. Dissolved Solids (<0.02 mm) 2 1 3

  10. The turbidity of a water treatment plant effluent cannot be above? • 5 ntu • 1 ntu • .5 ntu • .3 ntu

  11. Turbidity • Turbidity/Colloids– negative charged particles particles (sand, silt, clay, organic matter) in the initial source water that need to be removed to improve treatment.

  12. Coagulation • Coagulants tend to be positively charged.  • Due to their positive charge, they are attracted to the negative particles in the water • The combination of positive and negative charge results in a neutral , or lack, of charge • Van der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge. 

  13. Settling Forces Zeta Potentialrefers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle. It can help you understand and control colloidal suspensions van der Waals Force-van der Waals forces are weak attractive forces that hold non-polar molecules together.They tends to pull molecules together and forms flocs. Zeta potential analyzer van der Waals in action

  14. Settling Forces Zeta Potential -is the electrical potential that exists at the "shear plane" of a particle, which is some small distance from its surface. Keeps particles apart and in suspension

  15. Water Treatment Coagulants Particles in water are negative; coagulants usually positively charged. 1. Alum- aluminum sulfate 2. Ferric chloride or ferrous sulfate 3. Polymers

  16. What determines the optimum and most cost-effective amount of a coagulant to use?: • Beyond that dose, it takes a very large increase in the amount of chemical to produce a small increase in turbidity removal • Below that dose the coagulant results in poor settling • The treatment plant budget • Divide the number of gallons of water in the coagulation tank by the nephelometric turbidity unit reading to determine the dosage in mg/L.

  17. Which is NOT a common method for determining optimum coagulant effectiveness?: • Jar test • Zeta potential detector • Streaming current detector • Colorimetric method

  18. Water Treatment Coagulant Alum Alum- (aluminum sulfate)- particles suspended in natural, untreated water normally carry a negative electrical charge. These particles are attracted to the positive charges created by aluminum hydroxides. Dosage is generally around 25 mg/L. 1. Trivalent Al+3 charge attracts neg – particles 2. Forms flocs of aluminum hydroxide (AlOH3). 3. Impacted by mixing, alkalinity, turbidity and temp. 4. Ideal pH range 5.8-8.5

  19. Alum CHEMISTRY Alum- (aluminum sulfate)- made by dissolving aluminum hydroxide (bauxite or clay) in sulfuric acid 2Al(OH)3 + 3H2SO4 + 10H2O → Al2(SO4)3·16H2O When ALUM is dissolved in alkaline water, it undergoes hydrolysis (reacts with water) to produce a high surface area gelatinous precipitate of aluminum hydroxide, Al(OH)3 (gibbsite) (Al(OH)3sticks the negatives. When ALUM is reacted with water it hydrolyzes to form aluminum hydroxide and dilute sulfuric acid (lowers pH). -----Need alkalinity adjustment

  20. Alum CHEMISTRY Alum- (aluminum sulfate)- Al2(SO4)3·14H2O 2Al+3 +3SO4-2 +14H2O 2Al+3 + negatively charged colloids neutral surface charge WHY IS ALKALINITY SO IMPORTANT?? 2Al+3 + 6 HCO3- 2(Al(OH3)(S) + 6CO2 No bicarbonate (low alkalinity, low pH sulfuric acid!): Al2(SO4)3·14H2O 2(Al(OH3) (S) +3H2SO4-2 +14H2O Optimum pH: 5.5 to 6.5 Operating pH: 5 to8

  21. Alum MSDS Safety and Handling Alum- (aluminum sulfate)- Health Rating: 2 - Moderate Flammability Rating: 0 - None Reactivity Rating: 1 - Slight Contact Rating: 2 - Moderate Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD; PROPER GLOVES Storage Color Code: Green (General Storage) Inhalation: Causes irritation to the respiratory tract. Ingestion: Causes nausea, vomiting and diarrhea. There have been two cases of fatal human poisonings from ingestion of 30 grams of alum. Skin Contact: Causes redness, itching, and pain. Eye Contact: Causes irritation, redness, and pain.

  22. When alum is added to water, a floc is formed from the combination of alum and a. alkalinity b. acid c. chlorine d. lime

  23. The precipitate formed by coagulation with alum is aluminum ________. • Bicarbonate • Carbonate • Hydroxide • Sulfate

  24. Adding Alum to water will cause the pH of the water to increase. • True • False

  25. Alum comes in dry grade as a minimum of 17.5% pure product, in liquid form it is 49% pure or 8.23% by weight Al2O3? • True • False

  26. The coagulation process will most likely improve when: • The hardness of the influent increases • The temperature of the influent decreases • The temperature of the influent increases • The alkalinity of the influent decreases

  27. Water Treatment Coagulant Iron Salt Iron salt- (Iron chloride or sulfate)- particles suspended in natural, untreated water normally carry a negative electrical charge. These particles are attracted to the positive charges created by Fe(III) salts. 1. Trivalent Fe+3 charge attracts neg – particles 2. Work over a larger pH range than alum 3. Lower costs than alum 4. Better removal of natural organics 5. Corrosive 6. Special handling necessary 7. Leave a residue of Fe in water (taste, stains)

  28. Which one of the following chemicals would you most likely use as a coagulant?: • Cationic polymer • Sulfuric acid • Hydrochloric acid • Sodium hydroxide

  29. A coagulant aid is a chemical added during coagulation to improve coagulation; to build stronger, more settleable floc; to overcome the effects of temperature drops; to reduce the amount of coagulant needed, and/or to reduce the amount of sludge produced. Which of the following is not a type of coagulant aid: • Activated silica • Green sand • Polyelectrolytes or polymers • Weighting agents

  30. A microfloc is a colloid that has reacted with a chemical coagulant? • True • False

  31. In determining the proper dosage of alum, the most useful test is the _______ test: a. marble b. jar c. carbonate d. pH

  32. Alum added to turbid water containing alkalinity forms _________, which increase in size and settle out. a. floc particles b. coagulants c. coagulant aids d. aluminum sulfate

  33. Overcoming problems of cold-water floc can be corrected by operating the process at the best pH for that water temperature, increasing the coagulant dosage, or: • Adding weighting agents • Performing the jar test • Increasing the number and strength of floc particles • Increasing the detention time for floc formation

  34. Which of the following conditions most affect coagulation performance? a. velocity, chlorine dosage, detention time, and air temperature b. velocity, water temperature, detention time and coagulant dosage c. water temperature, detention time, air temperature, and chlorine dosage d. detention time, velocity, air temperature, and chlorine dosage

  35. With the coming of winter, the water temperature drops. A likely operational problem at a filtration plant with coagulation is: • Floc carryover from the sedimentation system • High chlorine residual • High alkalinity • Odor

  36. Water Treatment Coagulant Aids Activated silica (sodium silicate)- helps improve coagulation, decreases volume of coagulant necessary. Typically is sodium silicate. 1. secondary coagulant 2. reduces primary coagulants needed 3. Sodium silicate are alkaline 4. widens pH range for coagulation 5. used at 7-11% of alum 6. Heavier denser floc that settles faster 7. Can be formed on site 8. Corrosion inhibitor (forms a surface coating)

  37. The three most commonly used coagulants in water treatment are: • Aluminum hydroxide, lime and sodium hydroxide • Aluminum sulfate, ferric chloride, and ferrous sulfate • Lime, sodium hydroxide, and chlorine • Soda, lime and chlorine

  38. Water Treatment Coagulant Aids Bentonite (clay)- helps improve coagulation, decreases volume of coagulant necessary. 1. high in color, low turbidity, low mineral content water 2. 10-50 mg/L dosage 3. Heavier denser floc that settles faster

  39. Which of the following would most likely improve the coagulation/flocculation process? a. increase in raw water hardness b. decrease in water temperature c. increase in water temperature d. decrease in raw water alkalinity

  40. Water Treatment Coagulant Aids • Polyelectrolytes- are water-soluble organic polymers that are used as both primary coagulants and coagulant aids. Act as "bridges" between the already formed particles: • • Anionic—ionize in solution to form negative sites along the polymer molecule. • • Cationic—ionize to form positive sites. • • Non-ionic—very slight ionization. • effectiveness: particles type, turbidity present, and the turbulence (mixing) available during coagulation.

  41. Which one of the following chemicals would be most suitable as a filter aid? a. alum b. soda ash c. sodium hydroxide d. anionic polymer

  42. A high molecular weight substance that is formed by either a natural or synthetic process. Can have either positive or negative charge. • Polymer • Protein • Carbohydrate • Enzyme • Deoxyribonucleic acid

  43. Water Treatment Coagulant/pH • Alkalinity- Alkalinity is a measure of the buffering capacity of water. These buffering materials are primarily the bases bicarbonate (HCO3-), and carbonate (CO32-), and occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands. • Chemicals applied to raise alkalinity • • Lime—CaOH2 accompanies alum or iron salt • • Sodium bicarbonate- NaHCO3- raise alkalinity • Soda Ash—Na2CO3 -raise alkalinity • • Caustic Soda—NaOH -raise alkalinity

  44. Water Treatment WHO Coagulants

  45. Optimum flocculation requires: a. violent agitation b. gentle agitation c. high pH d. low pH

  46. Agglomeration of colloidal and finely divided suspended matter after coagulation by gentle mixing is called what? • Flocculation • Sedimentation • Polymer accretion • Ballasting

  47. Which laboratory test is concerned with indicator changes at pH 8.3 and about pH 4.5? a. total hardness b. pH c. alkalinity d. total chlorine residual

  48. _________ is the measure of how much acid can be added to a liquid, without causing a great change in pH. • Alkalinity • Hardness • pH • Acidity

  49. When operating a surface water plant, which laboratory tests are most significant for establishing dosages for coagulation? a. pH and alkalinity b. sulfates c. calcium and magnesium d. total hardness

  50. A test that is commonly performed to monitor the treatment process is : • pH • Alkalinity • Turbidity • All of the above